1. Field of the Invention
This application is directed to a subsurface safety valve system for use in drilling oil or gas wells. Such valves are commonly used to prevent flow of oil or gas from the well to the surface when certain conditions occur.
2. Description of Related Art
Currently such safety valves are held in an open position by virtue of pressure in a control line from the surface acting on a piston in the valve which is operatively connected to a flow sleeve which moves axially to open a valve member. Movement of the sleeve also compresses a spring surrounding the flow sleeve.
Upon the occurrence of an unfavorable event, the pressure is relieved via the control line so that the spring will move the flow sleeve upwardly so as to allow the valve, which may be a flapper valve to close. In so doing, the spring must overcome the pressure head caused by the hydraulic fluid and the flow resistance due to the small diameter of the control line.
Some control lines in deep water subsea wells may be up to two miles or more in length and may extend a vertical distance of more than a mile.
Consequently the pressure head and resistance to flow is quite high which can delay the response time for the valve and may in some cases result in failure.
FSSD—or fail safe setting depth is a term known to all skilled in the art of Surface Controlled Subsurface Safety Valves (SCSSVs) and is discussed in detail in API-14A, the primary document controlling certification of all such valves.
Simply put, the FSSD is the depth at which a SCSSV may not be set below because the force caused by the pressure head of a column of fluid in the control line from the surface acting on the valve's actuating piston is greater than the force of spring acting to close the valve.
In deep set valves, it is impossible to employ a spring large enough to close the valve so a gas charge, normally nitrogen, is commonly used to offset a portion of the force of the pressure head, thereby allowing the valve to operate somewhat normally. In the nitrogen chamber, often a low lubricity oil is positioned between the piston seals and the nitrogen to protect the piston seals, and to reduce the effects of wear as the piston cycles repeatedly between open and closed. The term “somewhat” is used here due to the compressible nature of gasses.
Pressure Charged SCSSVs actually have a “Fail Safe Setting Window” which is not absolute because of the changing nature of the downhole environment and its own particular wear characteristics. Normally deep set SCSSV's are utilized in deep ocean environments where temperatures are near freezing—33-40 degrees F. (or 1-3 degrees C.). SCSSVs are typically set 100 meters below the mud line of the ocean floor, and are influenced by these temperatures. The temperature of the producing formation can be 300-400 Fahrenheit, meaning the SCSSV can warm to these temperatures during production of the well. However, if the well is shut-in the temperature can rapidly cool to that of the ocean floor.
The result is that in a constant volume chamber, the pressure changes dramatically with temperature in application of Boyle's and Charles' Law: P1/T1=P2/T2. Therefore, in a gas charged SCSSV, as the nitrogen chamber warms to, for example, 350 deg F., the nitrogen is able to offset a greater pressure head than when it has cooled to 33 Deg F. during shut in. Over time, repeated open and closing cycles cause minute longitudinal scratches in the piston bore and on the seals thereby allowing small amounts of oil to leak past the seals. With enough cycles, the seals can fail causing the nitrogen to leak off, triggering a highly complex valving system to auto-execute to prevent failure of the valve in the open position—if it works properly—and the above described “non-fail safe” scenario has not happened. This is a characteristic and risk assessment associated with all prior art deep set SCSSVs.
What is known to all SCSSV designers is that reducing the piston area increases FSSD. Obviously, the opening force exerted by the control line fluid is equal to the pressure head times the piston area. As piston area approaches zero, FSSD approaches infinity.
However, until the present invention there has always been a practical limitation of piston diameter. When the valve is closed the operating piston exists happily completely enclosed in the piston bore. However, as the valve opens, the piston strokes out of the bore and extends itself as a cantilevered beam until the valve is open. The length of the cantilevered piston is always greater than the flapper diameter, as it must push the flow tube to fully open the flapper.
The cantilevered piston has two possible loading conditions; the first as a column, as the power spring places compressive force on the unsupported piston; the second in bending, as the repeated cyclic compression of the spring places a radial load on the cantilevered piston, AND the combination of both of these loads. The piston resists these forces by the yield strength of the material and its Moment of Inertia. Designers already use the strongest, most noble materials known. The problem is reducing Moment of Inertia by reducing diameter. If the piston gets too long and skinny, it will fail due to elastic instability, bending, or both.
For this reason, most pistons have a practical diameter of ½ or ⅜ of an inch. In small tubing sized valves, pistons have been known to be ¼ inch.
The short length of the micro piston in accordance with the instant invention allows practical diameters below ¼ of an inch and practically can be used at diameters of 0.100 inches or even 0.050 inches. The stroke of the micro-piston to release the flow tube is very small as well, as an example less than 1 inch. This means the micro-piston has much lower wear characteristics, may be used at depths of 15,000 feet or even deeper without a gas charge, and is virtually unaffected by gas accumulation in the annulus. The micro-piston, because of its short stroke, may also cycle 20,000 or 30,000 times before predicted failure.